Fix __repr__, move it to the base component

[rmarren1]

This does two things at once:

1. Fixes some issues in __repr__
2. (proposed) Move __repr__ to the base component, as there is really no reason it needs to be part of the generated Python classes.

I realize now that 2 has the slight issue that a new component using an old version of dash will not have its own __repr__ function, and it will not inherit one from the base component, so the resulting __repr__ will be the default.

We have required updated component libraries to update dash before, e.g. here, but I think these occurrences should be rare, so I'm not really sure if we should also do 2 here and perhaps I should delete it and just do 1.

@T4rk1n please review 1, also do you have an opinion on whether we should do 2? (I would also have to re-build core component libraries with and add version checking to core libraries)

[T4rk1n]

__repr__ should be defined in the base class.

[rmarren1]

Okay, then I'll add something along the lines of

if not _dash.version.__version__ > <version_with_repr>:
    print("Please update the `dash` module to >= <version_with_repr> to use this "
          "version of dash_html_components.\n"
          "You are using version {:s}".format(_dash.version.__version__),
          file=_sys.stderr)

to the current component libraries

[T4rk1n]

😸

[T4rk1n]

Looks good, 💃

[alexcjohnson]

@T4rk1n if you still want this PR included, please resolve conflicts and merge it.